1. Field of the Invention
The present invention relates to a rotation sensor device for detecting a rotation state (i.e., rotation speed or rotation number) of a rotating magnetic member formed in a shape such as, for example, that of a toothed wheel. The invention is further concerned with a method of manufacturing such rotation sensor device.
2. Description of Related Art
For having a better understanding concerning the technological background of the present invention, description will first be directed to a conventional rotation sensor device known heretofore.
FIG. 7 is a sectional view showing a conventional rotation sensor device. Referring to the figure, the rotation sensor device generally denoted by a reference numeral 14 is comprised of a rod-like magnetic core 4 and a coil 2 wound around the magnetic core 4 with a bobbin 3 being interposed therebetween, wherein outer peripheral surface of the coil 2 as a whole is covered with a casing 1 which is formed of a resin by molding. The magnetic core 4 has a tip end portion which projects outwardly from the casing 1 and Which is disposed in opposition to a toothed-wheel-like rotatable magnetic member (also referred to as the toothed magnetic disk) 100 which is mounted on, e.g. a shaft whose rotation state is to be detected. Disposed on the other end portion of the magnetic core 4 are a bias magnet 5 and a spacer 6 for completing a magnetic circuit. The bias magnet 5 and the spacer 6 are fixedly held in place by means of a grommet 7 which is made of a rubber. Wires (not shown) led out from the coil 2 are connected to terminal fittings 8 which in turn are connected to an external circuit by way of lead wires 10 and a connector 12. A rear end portion of the rotation sensor device 14 is enclosed by a cover 9. An O-ring 11 is provided for ensuring dust-proof installation of the rotation sensor device.
FIG. 8 shows in a block diagram an apparatus for measuring the rotation speed of the toothed magnetic disk 100 on the basis of the output of the rotation sensor device 14. Referring to the figure, the rotation sensor device 14 is connected to a computer unit 17 via a filter circuit 15 provided for noise elimination and a Schmitt trigger circuit 16 which is composed of comparator and switching circuitries.
Next, operation of the apparatus described above will be considered. By virtue of such arrangement of the rotation sensor device 14 that the magnetic core 4 and the spacer 6 are disposed closely adjacent to both lateral surfaces of the bias magnet 5, respectively, as can be seen in FIG. 7, and that the coil 2 is wound around the magnetic core 4, there is induced in the coil 2 an AC voltage in accompanying the rotation of the toothed magnetic disk 100 disposed in the vicinity of the free end of the magnetic core 4. The AC voltage as induced is then outputted onto the lead wire 10 connected to the external circuit.
More specifically, the AC output voltage is supplied to the filter circuit 15 to undergo noise elimination and thereafter converted into a voltage signal of a pulse-like waveform by the Schmitt trigger circuit 16. The computer unit (such as microcomputer) 17 calculates the period of the pulse signal (or counts the number of pulses) to thereby arithmetically determine the rotation number or speed (rpm) of the rotatable magnetic toothed disk 100.
The conventional rotation sensor device as well as the rotation speed measuring system described above suffers from a problem that when the toothed magnetic disk 100 whose rotation state is to be detected is rotating at a low speed, it becomes difficult to detect the rotation of the disk 100 with reasonably high accuracy and reliability.
More specifically, as is known as the Faraday's law of electromagnetic induction, the electromotive force induced in the rotation sensor circuit by a changing magnetic field generated by the toothed magnetic disk 100 is equal to the negative of the rate of change of the magnetic flux linkage for the circuit. Thus, when the rotation speed of the toothed magnetic disk 100 becomes low, magnitude of the output voltage of the rotation sensor device decreases proportionally, which in turn means that the S/N ratio (signal-to-noise ratio) of the sensor output signal becomes small, to incur degradation in the accuracy of the detection of the rotation speed.